There is a need to provide a gas-fired hot water heater with explosion proof protection in the event that flammable vapors, propagating on a floor surface on which the hot water heater is supported, reach the combustion chamber and ignite the vapors causing explosion. In my U.S. application Ser. No. 10/234,140 filed on Sep. 5, 2002 and entitled “EXPLOSION PROOF GAS-FIRED WATER HEATER”, there is described a gas-fired heater with a sealed combustion chamber and wherein combustion air is supplied to the combustion chamber through a vertical duct which is in sealed communication with the combustion chamber and which has an elevated air inlet opening located well above the support surface of the hot water heater. A flammable gas vapor detector is associated with that hot water heater whereby to detect flammable vapors close to the floor area of the hot water heater and to shut off the supply of gas to the burner and pilot well in advance of the flammable gas vapors rising to the inlet port of the duct. It is also known to support a gas water heater on a support base to elevate the combustion chamber at least eighteen inches above the support floor. It is also known to provide flame arrestors in the bottom wall of a combustion chamber to resist to flammable vapors. These structures require reconfiguring the bottom wall of the combustion chamber, see U.S. Pat. No. 6,295,952 issued on Oct. 2, 2001.